Wireless communications offer increased convenience, versatility, and mobility compared to wireline alternatives. Cellular phones, wireless computer networking, and wireless peripheral components, such as a mouse, headphones, and keyboard, are but a few examples of how wireless communications have permeated daily life. Countless additional wireless technologies and applications are likely to be developed in the years to come.
Wireless communications use various forms of signals, such as radio frequency (RF) signals, to transmit data. A transmitter broadcasts a signal from an antenna in a particular frequency band. As the signal travels, the signal loses power or attenuates. The farther the signal travels, the more the signal attenuates.
The signal also encounters various forms of interference along the way that introduce noise in the signal. The transmitter itself introduces noise. Signals from other transmitters also introduce noise. A receiver trying to receive the signal is likely to introduce a comparative large amount of noise. Virtually anything can cause noise, including the ground, sky, sun, and just about any animate or inanimate object.
At some distance from the transmitter, the signal will attenuate to the point that it becomes lost in noise. When noise overpowers a signal, the signal and the data it is carrying are often unrecoverable. That is, depending on the distance a signal travels and the amount of noise mixed with the signal, a receiver may or may not be able to recover the signal.
Of particular concern is noise introduced in a receiver by a transmitter that is located in close proximity. The noise is called a coupled signal. A coupled signal may introduce so much noise that the receiver cannot receive any other signals. Signal coupling is a major obstacle in wireless communications. Devices that include both a transmitter and a receiver must isolate the transmitter from the receiver so that the coupled signal does not overpower the receiver.
One approach to isolating a transmitter from a receiver is half duplex communications. A half duplex device cannot simultaneously send and receive. A common example is a hand-held, two-way radio. When a user pushes a button to talk into the radio, the user cannot simultaneously listen to signals from other radios. That is, the receiver is disabled when the transmitter is transmitting. If the receiver were not disabled while the transmitter transmits, the transmitter would probably over power the receiver with noise.
Isolation is particularly troublesome in devices that include more than one on-board radio. For instance, a portable computer may include more than one radio to enable more than one simultaneous wireless service. A transmission from any one radio may over power receivers in multiple radios. One approach to isolating multiple transmitters from multiple receivers is time division duplex (TDD) communications. In a TDD device, all receivers are disabled when any one transmitter transmits.
A cellular phone, on the other hand, is a full duplex wireless communication device. That is, a cellular phone simultaneously transmits and receives signals so that a user can talk and listen at the same time. A cellular phone isolates its transmitter from its receiver by using two different frequency bands—one band for transmitting and one band for receiving.
None of these isolation solutions are particularly satisfying. Half duplex and TDD communications have the obvious disadvantage that a user cannot simultaneously send and receive. This poses a substantial performance limitation that will become more pronounced as more wireless communications applications and technologies are developed and adopted, and more devices include multiple on-board radios.
Full duplex communications that rely on two isolated frequency bands for sending and receiving data have the obvious disadvantage of using twice as much frequency bandwidth as half duplex communications. This poses a substantial performance limitation that will also become more pronounced as the numbers of competing wireless applications and users continues to increase, and available bandwidth continues to decrease.